#include<iostream>

using namespace std;
#define int long long
int l , n , m;
const int N = 50010;
int a[N];

int check(int x)
{
    int ret = 0;
    for(int i = 0;i <= n;i ++)
    {
        int j = i + 1;
        while(j <= n && a[j] - a[i] < x) j++;

        ret += j - i - 1;
        i = j - 1;
    }
    return ret;
}
signed main()
{
    cin >> l >> n >> m;
    for(int i = 1;i <= n;i ++) cin >> a[i];
    a[n + 1] = l;
    n ++;

    int left = 1 , right = l;
    while(left < right)
    {
        int mid = (left + right + 1) / 2;
        if(check(mid) <= m) left = mid;
        else right = mid - 1;
    }

    cout << left << endl;
    return 0;
}